EES215

Lecture 21

Review of energy at surface of earth
Use of energy: main sources still from fossil fuels: coal, oil and natural gas. Example: USA (Fig. 1 and Fig.2), typical for distribution and history of energy sources in the world
Differences between energy use: World average - 2 kW/person; US average - 10 kW/person
Populations: World: 6x10⁹ people; USA - 300x10⁶ people (roughly).
Total energy use: World 2,000 W/person x 6x10⁹ people x 3.15x10⁷s/yr = 3.78x10²⁰ J/yr.
US population (5 % of world population) consumes 25 % of total energy in the world.

Energy from fossil fuels is derived from the following reactions:

Coal:
C + O₂ => CO₂

Natural Gas (mainly methane, CH₄):
CH₄ + 2O₂ => CO₂ + 2H₂O

CO₂ release from coal:
Energy content of coal: 7300kWhr/ton = 2.64x10¹⁰ J/ton
Roughly one third of energy comes from coal (world): 3.78x10²⁰ J/yr x 0.3 = 1.13x10²⁰ J/yr
Total coal used: 1.13x10²⁰ J/yr / 2.64x10¹⁰ J/ton = 4.29 x 10⁹ tons of coal/yr

C in CO₂: 12/(12+2x16) = 0.27
CO₂ released from burned coal: 4.29 x 10⁹ tons of C/yr / 0.27 = 1.6 x 10¹⁰ tons of CO₂/yr

CO₂ release from natural gas (CH₄ ):
Energy content of gas: 3.68x10⁷ J/m³ ; density of gas: 0.6 kg/m³ à 6.13x10⁷ J/kg
Roughly one third from total energy comes from gas: 1.13x10²⁰ J/yr
Total methane used: 1.13x10²⁰ J/yr / 6.13x10⁷ J/kg = 1.84x10¹² kg of CH₄/yr (or 3x10¹² m³/yr)

C in CH₄: 12/(12+4x1) = 0.75 à 1.84x10¹² kg x0.75 = 1.38x10¹²kg of C/yr
CO₂ into atmosphere: 1.38x10¹²kg/0.27 = 5.12x10¹²kg/yr from natural gas

Release from oil (not shown here) ~8x10¹² kg/yr

Total annual release of CO₂ into atmosphere : 1.6x10¹³ kg + 5.12x10¹² kg + 8x10¹² kg = 2.9x10¹³ kg

Comparison to CO₂ in atmosphere:  CO₂ in the atmosphere has increased from 320 to 380 ppm(v) since 1958. The increase is parallel in both hemispheres, but the seasonal fluctuations are more pronounced in the Mauno Loa record (Fig. 3) than in the record from the South Pole (Fig. 4). The increase is roughly linear, i.e. it increased by 60/320 = 0.188 over 50 years or 0.4 % per year.

Total amount of CO₂ in the atmosphere: ppm(v) à ppm(m) for CO₂ = 1.5 à 380 ppm(v) à 570 ppm(m)

Mass of atmosphere: 5.18x10¹⁸ kg

Mass of CO₂ in atmosphere 5.18x10¹⁸ kg x 570x10^-6à 2.6x10¹⁵ kg

Annual observed increase: 2.6x10¹⁵ kg x 0.004 = 1x10¹³ kg, i.e. less than half of the CO₂ released from fossil fuel burning remains in the atmosphere, a large fraction of the remainder finds its way into oceans. There are still important questions open about the role of the biosphere and complete accounting for the CO₂ released from fossil fuel burning.

The greenhouse problem
Solar spectrum; shift to different radiation - short wave length to long wave length ( Fig. 5 ); absorption behavior of main constituents of atmosphere: Fig. 6
Greenhouse gases: CO₂; CH₄; H₂O; SO_x etc.

Greenhouse effect:  With the increase of greenhouse gases in the troposphere, an increasing amount of the outgoing, infrared radiation is kept in the lower atmosphere, causing long-term warming Fig. 7